II John and III John are letters to individuals.. the first to a church, addresses as "the chosen lady and her children" and the second to Gaius.   Both letters encourage the believers to keep walking in the truth.  The churches were being targeted by false teachers who were denying that Jesus had come in the flesh.  Over and over John stresses how vital it is to cling to the truth. 

II John
John loves the church "in truth". v1
All who "know the truth "  love the church also.
"the truth which abides in us and will be with us forever"  - Jesus ! v2
"Grace, mercy and peace will be with us, from God the Father and from Jesus Christ the Son of the Father, in truth and love." v3

To find the believers "walking in truth" brings joy to John. v4
Those walking in the truth will obey the Lord's commandments.. loving one another and  abiding in the teaching "of Christ". (v6, 9)

III John

"I have no greater joy than this, to hear of my children walking in the truth." v4  John writes to his beloved friend.
Witnesses came to John and testified that Gaius was:
"acting faithfully" (v5)
showing "love" for the brethren. (v5-6)
supporting and sending out those  who "went out for the sake of the Name."(v6-8)

In contrast, Diotrephes, who was also in the church, does not appear to be walking in the truth.
He "loves to be first" (v9)
He "does not accept" what John or the apostles say.
His "deeds" are evil. ( v10-11)
He " unjustly " accused the apostles "with wicked words". (v10)

"Beloved do not imitate what is evil, but what is good. The one who does good is of God; the one who does evil has not seen God." (v11)

One more example is given , that of Demetrius. ( v12)
Demetrius "has received a good testimony from everyone, and from the truth itself."
John also bears witness of Demetrius' good walk.

Our deeds and words... bear witness of what is in our heart.  If we are walking in the truth then our deeds and words will testify that we are "of God".  We will be like Gaius... faithful, loving, generous towards God's ministers of the truth.  If we are not walking in the truth we will be more like Diotrephes, self-centered, proud, wicked.

We must be careful to keep abiding in Jesus, who is the Truth, and who lives in us forever and ever!
We must also be discerning, to "watch ourselves" so that we don't imitate what is evil, but only what is good.

Lord, keep us walking in the truth.  Keep our hearts pure and our minds and wills humbly yielded to Your Spirit's leading in the way of Jesus, our Savior.  Thank You. Amen.